Advance ticket sales for the new Harry Potter film are outpacing those of Transformers: Revenge of the Fallen in both the UK and the US, suggesting it could be on course to become the biggest film of the year at the global box office.

Michael Bay's action blockbuster about warring races of alien robots currently tops the 2009 box office chart. It passed $300m (£186m) in the US today, becoming the first film of the year to do so, and has taken more than £20m in the UK, so far.

Odeon and UCI cinema group, which is the largest in the UK, said it had sold in excess of 76,000 advanced tickets for the sixth Harry Potter film, equating to more than £500,000 in box office takings. So far, the Transformers sequel has taken £8.35m

Vue cinemas, which owns 66 sites, with a total of 641 screens in the UK and Ireland, said The Half-Blood Prince was well ahead of the previous Potter movie, Order of the Phoenix, at a similar point in the sales cycle, although it refused to comment on whether the film was ahead of Transformers: Revenge of the Fallen

On those sort of figures, The Half-Blood Prince could well break into the all-time UK top 10, although it will have a long way to go before it topples the No 1 film, last year's Mamma Mia!, which racked up a staggering £69.17m. The Abba musical benefited from huge numbers of repeat viewings, as did the second-placed film, Titanic, which is just behind on £69.03m. The highest-placed Potter film, Harry Potter and the Philosopher's Stone, is third, with £66.10m, while Order of the Phoenix lies in 13th spot with £49.43m.

In the US, The Half-Blood Prince is currently accounting for 65% of online ticket sales, according to retailer Fandango. The film, which sees teenage wizard Harry discover a mysterious schoolbook whose previous owner's annotations help him excel in his potions class, opens on both sides of the Atlantic on 15 July. The sixth big-screen adaptation of JK Rowling's tales is also ahead of the Order of the Phoenix in the US.

In total, the Harry Potter movies have taken more than $2.77bn at the global box office. In terms of film franchises, only Eon Productions' James Bond has made more money - £3bn - but from 22 films.

Charles Gant: The hormonal Hogwarts hero has earned more at the British box office than the Transformers can wave a wand at. And with no challenger in the pipeline, the wizard is set to be this summer's undisputed blockbuster champ